Avoid updating the L2ARC device header unnecessarily

If we do not write any buffers to the cache device and the evict hand
has not advanced do not update the cache device header.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #11522
Closes #11537